What a conductor does

Almost all of that work happens in a rehearsal room. What the audience sees on the night is the smallest and least difficult part of the job, which is why the question is so often answered with a joke about waving a stick.

The rehearsal week, where the job actually happens

The rehearsal week is where a conductor earns the engagement. A professional orchestra prepares a standard programme in about three to four rehearsals of three hours, and much less than that on tour, so roughly nine to twelve hours of collective time exists to turn a stack of parts into a performance. That time is the most expensive thing an orchestra owns. A conductor who arrives without having decided the tempi, the balance and the bowings will spend it discovering them in front of 90 people who already know the piece.

Before the first rehearsal the conductor has chosen the edition, agreed the string bowings with the leader, planned the seating, and worked out a schedule that puts the hardest passages where the players are freshest and releases the brass and percussion when they are not needed. A rehearsal hall is hired by the hour and the orchestra is paid by the session. Nothing about the calendar is casual.

What happens in a first rehearsal

A first rehearsal usually begins with the whole first movement played straight through, without stopping, so that the conductor can hear the orchestra's own habits and the orchestra can hear the conductor's tempo. Then the work is taken apart. The conductor stops, names a bar number, asks for something specific and small, and runs the passage again. Good rehearsal technique is measured in what is not said: every stop costs a minute of playing time, and a conductor who explains at length is taking that minute from the ensemble.

The corrections themselves are unglamorous and concrete. Second horn is a semiquaver late into the bar. The cellos are playing a printed forte that swallows the bassoon line underneath it. The woodwind chord at a transition is not tuning because the oboe is leading it and the clarinet is listening to the flute. None of this is interpretation. It is the mechanical assembly that has to be finished before interpretation is worth discussing.

Rehearsal work and concert work compared

The split between the two halves of the job is sharp enough to set out in full. Everything in the left-hand column is negotiable and repeatable. Everything in the right-hand column happens once.

The decisions nobody else in the room can make

Certain decisions belong to the conductor because no other single person is in a position to take them. There are five that recur in almost every programme.

• Choose the tempo, including the tempo of a passage the composer marked only with a word such as Andante, which has meant different speeds in different centuries.
• Decide the balance, because players cannot hear the hall from the platform and a chord that is correct on stage is often wrong in the stalls.
• Settle the edition and the cuts, which in a work such as Rachmaninov's Second Symphony of 1907 is the difference between about an hour of music and a heavily trimmed version.
• Fix the transitions, the places where one tempo becomes another, since these cannot be negotiated between sections that are ten metres or more apart.
• Time the silences, including the length of a pause between movements and how long a final chord is held before the hands come down.

Everything else is shared. The leader settles the bowings for the strings, section principals decide phrasing inside their own lines, and an experienced orchestra will fix most internal ensemble problems if simply left alone for a bar or two.

What happens if the conductor stops beating

A good professional orchestra keeps playing, and for a while it keeps playing well. Inside a steady passage at a settled tempo, the players are listening to each other and watching the leader, and the beat is largely confirmation of something already running. This is the honest basis of the joke, and conductors know it better than the audience does.

The point at which it fails is predictable. It fails at any change of tempo, at an entry that follows 30 bars of rest, at a rallentando where every player has a different instinct about how much to slow down, and at the start of anything. It fails hardest in choral works, where several hundred amateur singers, a chorus master's preparation and an orchestra all have to arrive together: the Huddersfield Choral Society, a lay society founded in 1836, and the great oratorio repertoire depend on that far more than a Haydn symphony does. And it fails at the moment of maximum exposure, which is the first sound of the evening.

Youth orchestras, where the work is most visible

In a youth orchestra the same job is done in the open, because nothing can be assumed. Players are auditioned each year, the personnel changes, and a course typically compresses a whole programme into a residential week rather than three or four rehearsals. The National Youth Orchestra of Wales, founded in 1945 and the first national youth orchestra in the world, works on that pattern. Owain Arwel Hughes was its music director from 2003 to 2010, one of a series of posts across his career that also included associate conductor of the BBC Welsh Orchestra from 1980 to 1986.

Watching a conductor with a youth orchestra is the clearest available demonstration of the job, because the difference the work makes is audible across a single week rather than hidden inside a professional ensemble that would have played the piece competently anyway. The measure of a conductor is not what happens when the beat stops. It is what the orchestra sounds like on Saturday compared with Monday.

Also in this programme

• ConductingWhat conducting is, and what it is for
• Famous orchestral conductorsThe conductors most people can name, and why
• The baton and the beatWhat it is for and how the beat is shaped
• The leaderThe leader of the orchestra, and what a concertmaster does